The direction the clubface points — left or right of the target line — at the moment of impact. Launch monitors measure it at maximum ball compression: a positive number means the face is open (aimed right of target for a right-hander), negative means closed (aimed left). It is the single biggest influence on where the ball starts; the ball launches very close to wherever the face is pointing, with the swing path playing a smaller supporting role. That's why a shot can start left even when you swung out to the right. On the sim screen, 0.0° is a face dead square to the target — though shot-shapers deliberately deliver a degree or two either way.